In a data processing using a computer, a parallel processing technology is generally well known. Patent literature 1 discloses a technology for performing a parallel processing in which a new idea about a hash join is created. In the technology disclosed in patent literature 1, the hash join is divided into a plurality of tasks on the basis of a frequency of occurrence of a join key of a table and a plurality of the tasks are assigned to a plurality of computers. In the technology disclosed in patent literature 1, in a case in which the frequency of occurrence of the join key is biased and whereby, the processing amounts of the tasks are not uniform, the task with a large processing amount is divided into a plurality of small sub tasks. In the technology disclosed in patent literature 1, the task and the sub task are assigned to a plurality of computers so that the processing amounts of the computers are equal to each other.